Vibronic measuring devices are known to be used, among other things, for determining the reaching of a predetermined fill level or for monitoring a minimum or maximal fill level of liquids or bulk goods in a container. For determining whether a certain limit-level has been reached, the vibronic measuring device is placed at the corresponding height in the container.
Vibronic measuring devices have, as a rule, two rods, which are arranged like the tines of a fork and which are excited via a membrane of a drive unit to opposite phase oscillations at the resonance frequency. The drive is provided, in such case, via piezoelectric elements. If the oscillatory system is covered by the measured medium, then the oscillation is attenuated, wherein, in the case of bulk goods, essentially the amplitude change is evaluated and, in the case of liquids, essentially the frequency change is evaluated. Such vibronic measuring devices are available from Endress +Hauser under the marks, LIQUIPHANT (for liquids) and SOLIPHANT (for bulk goods). The construction of a Liquiphant measuring device is described, for example, in EP 1261437 B 1.
Also possible with such vibronic measuring devices is the determining of density of the measured medium. The higher the density of a liquid, the smaller is the resonance frequency. The resonance frequency has, however, a temperature dependence, so that, for an exact density determination, also temperature of the medium must be determined. Moreover, various types of applications are known, in the case of which, besides the fill level, also the process temperature must be determined.
Over the years, temperature measurement has been possible, for example, by introducing an additional temperature sensor externally, i.e. outside of the fill level- or density measuring device, via a separate process connection into the container. For a temperature compensated density measurement, the temperature sensor is connected, together with the density measuring device, to the evaluation computer. A disadvantage of using two separate measuring devices is that each additional process connection, besides increasing costs, also presents a supplemental risk as regards sealing and the hygiene of the process.
A mounting of the temperature sensor directly on the membrane or on the oscillatory fork is, for mechanical reasons, not possible, since the oscillatory system is negatively affected thereby as regards its functional ability. An integration of temperature sensor into the housing of the fill-level measuring device is difficult because of the type of mounting of the piezoelectric driver/receiver unit. This is from the side of the housing facing away from the oscillatable unit using a pressure-exerting screw unit. If the temperature sensor is secured on the housing wall, it represents an impediment to the screwing of the screw unit.
Known from DE 102006007199 A1 is a vibration limit switch arrangement, in the case of which a temperature determining unit is placed, for example, in the region between the oscillatable unit and the transmitting/receiving unit. It is not shown how the coupling between transmitting/receiving unit and oscillatable unit occurs across the temperature sensor, so that one wonders how the transmission of the mechanical oscillations to the oscillatable unit can occur unimpaired.